JP-A-5-88767 (Patent Document 1) discloses a band-gap reference circuit designed so that a bias current is supplied from two-stage current-coupled current mirror circuits to a band-gap reference generating circuit. Furthermore, JP-A-6-180616 (Patent Document 2) discloses a band-gap reference voltage generating circuit that includes a constant voltage output portion in which a power supply terminal to be supplied with current is connected to a reference voltage output terminal for reference voltage output and a constant voltage is output to the power supply terminal. The band-gap reference voltage generating circuit also includes a load-variable current supply portion having an emitter follower transistor in which the emitter is connected to the reference voltage output terminal for supplying current thereto, and a base potential controller for negatively feeding back the potential variation of the reference voltage output terminal to the base of the emitter follower transistor.
Enhancement of the vehicle performance by electric control as well as the addition of various functions for providing user convenience have greatly increased the number of electric control units (hereinafter referred to as ECU) mounted in a vehicle. The ECU comprises a microcomputer as a main body and is equipped with a main power source for operation and a power source for backup of RAM. As the scale of the system is larger, the consumption current of the overall ECU when an ignition switch is turned on is increased, and also the operating current (standby current) of the power source for backup, etc. when the ignition switch is turned off is increased. The increase in current consumption causes a decrease in the battery lifetime.
A power source circuit for backup is constructed by a band-gap reference voltage generating circuit, an output voltage detecting circuit, an error amplifying circuit and a constant current circuit, etc. In order to reduce the operating current, it is required to reduce the operating current of not only the band-gap reference voltage generating circuit, but also the other respective circuits.
FIG. 5 shows the electrical circuit construction of a band-gap reference voltage generating circuit disclosed in Patent Document 2. This band-gap reference voltage generating circuit 1 comprises a reference voltage producing circuit 2, an operational amplifier 3 and transistors Q1, Q2. Battery voltage VB is supplied from the terminals 4, 5 of the IC. A band-gap reference voltage VBG is output that has limited temperature dependence on the terminals (or internal nodes) 6, 7 of the IC.
The reference voltage producing circuit 2 includes a series circuit, which includes a resistor R1 and a diode-connected transistor Q3, connected to another series circuit, which includes a resistor R2, a transistor Q4 and a resistor 3 between the terminals 6 and 7. The bases of the transistors Q3 and Q4 are commonly connected to each other, and the voltage (reference voltage) of the common base line is connected to the base of input transistors Q5 of the operational amplifier 3. The collector voltage (reference voltage) of the transistor Q4 is connected to the base of input transistor Q6 of the operational amplifier 3.
The operating current flows through the series circuits of the reference voltage producing circuit 2 at all times. Therefore, in order to reduce the operating current (consumption current) of the band-gap reference voltage generating circuit 1, the resistance values of the resistors R1, R2 and R3 are increased to reduce the operating current. However, when the operating current is reduced, the band-gap reference voltage VBG is liable to vary in accordance with the variation of the battery voltage VB. Therefore, in the conventional construction, it is required to externally equip a capacitor between the terminals 6, 7 rather than increasing the resistance values of the resistors R1, R2, R3. However, the addition of a capacitor causes an increase in substrate area and associated costs.